(AP) - Declaring "our security is at stake,"President Barack Obama ordered an additional 30,000 U.S. troopsinto the long war in Afghanistan on Tuesday night, but

WEST POINT, N.Y. - (AP) - Declaring "our security is at stake,"President Barack Obama ordered an additional 30,000 U.S. troopsinto the long war in Afghanistan on Tuesday night, but balanced thebuildup with a pledge to an impatient nation to begin withdrawingAmerican forces in 18 months.

In a prime-time speech at the U.S. Military Academy, thepresident said his new policy was designed to "bring this war to asuccessful conclusion." The troop buildup will begin almostimmediately - the first Marines will be in place by Christmas - andwill cost $30 billion for the first year alone.

"We must deny al-Qaida a safe haven," Obama said inarticulating U.S. military goals for a war that has dragged on foreight years. "We must reverse the Taliban's momentum. ... And wemust strengthen the capacity of Afghanistan's security forces andgovernment."

The president said the additional forces would be deployed at"the fastest pace possible so that they can target the insurgencyand secure key population centers."

Their destination: "the epicenter of the violent extremismpracticed by al-Qaida."

"It is from here that we were attacked on 9/11, and it is fromhere that new attacks are being plotted as I speak," the presidentsaid.

It marked the second time in his young presidency that Obama hasadded to the American force in Afghanistan, where the Taliban hasrecently made significant advances. When he became president lastJanuary, there were roughly 34,000 troops on the ground; there noware 71,000.

A new survey by the Gallup organization, released Tuesday,showed only 35 percent of Americans now approve of Obama's handlingof the war; 55 percent disapprove.

In addition to beefing up the U.S. presence, Obama has askedNATO allies to commit between 5,000 and 10,000 additional troops.

He said he was counting on Afghanistan eventually taking overits own security, and he warned, "The days of providing a blankcheck are over." He said the United States would support Afghanministries that combat corruption and "deliver for the people. Weexpect those who are ineffective or corrupt to be heldaccountable."